This invention relates generally to sol-gel processes for producing dry gel bodies and, more particularly, to drying processes and apparatus for rapidly drying wet porous monolithic bodies at elevated, subcritical temperatures and pressures.
Sol-gel processes are gaining increased popularity in the creation of large, high-purity monoliths of glass and ceramic materials. In such processes, a desired solution, i.e., a sol, including glass- or ceramic-forming compounds, solvents, and catalysts, is poured into a mold and allowed to react. Following hydrolysis and condensation reactions, the sol forms a porous matrix of solids, i.e., a gel. With additional time, the gel shrinks in size and expels fluids from its pores. The wet gel is then dried in a controlled environment, to remove fluid from its pores, and it is then consolidated into a dense monolith.
Advantages of the sol-gel process include chemical purity and homogeneity, flexibility in the selection of compositions, processing at relatively low temperatures, and producing monolithic articles close to their final desired shapes, thereby minimizing finishing costs. Despite these advantages, the sol-gel process has generally been difficult to use in producing monoliths that are large and free of cracks. These cracks arise during the drying step of the process, and they are believed to result from stresses due to capillary forces in the gel pores. Efforts to eliminate the cracking problem present in sol-gel monoliths have been diverse. However, the problem of cracking has not previously been eliminated without adversely affecting one or more of the advantages, as listed above, or without incurring undue expense.
Sol-gel derived bodies have previously been dried using any of several distinctly different approaches. In one approach, the wet gel is heated above the critical temperature of the solvent being used as the drying medium, in an autoclave or drying chamber that permits the pressure to exceed the solvent's critical pressure. Above the critical temperature and pressure, there is no vapor/liquid interface in the pores, so no capillary force exists. Therefore, the shrinkage of the wet gel is negligible during drying. The solvent is removed from the pores while the critical temperature and pressure are exceeded, until the gel is completely dried. Although this "supercritical" drying technique is generally effective, providing an autoclave operable at the required temperatures and pressures (greater than 243.degree. C. and 928 psia in the case of ethyl alcohol) can be prohibitively expensive for large scale manufacturing. Operating at such high temperatures and pressures also can be dangerous.
Inorganic solvents, such as liquid carbon dioxide (CO.sub.2), also have been used as the drying solvent in an attempt to at least avoid the need to operate at excessively high temperatures. CO.sub.2 's critical temperature is 31.degree. C., and its critical pressure is 1070 psia. CO.sub.2 also is advantageously used because it is not explosive. However, the compression equipment necessary for liquefying gaseous CO.sub.2, and the cryogenic equipment necessary for maintaining CO.sub.2 in its liquid state, are very expensive. Consequently, CO.sub.2 is not believed to provide a commercially attractive alternative.
In an alternative approach, the wet gels are dried at ambient pressure (14.7 psia), and at temperatures close to or slightly higher than the boiling point of the solvent used as the drying medium. An example of this approach is provided in U.S. Pat. No. 5,243,769, to Wang et al. This approach, however, causes excessive shrinkage of the wet gel during drying, resulting in very small pore size dry gels.
In another approach, the gel is heated to such temperatures in a chamber having several pin holes through which the evaporating liquid escapes. Because the chamber is ventilated to the ambient environment, the pressure cannot increase above ambient pressure. Although this approach is generally effective, it can be very slow, at times requiring as much as a month or more to complete the drying process. The drying rate can be increased by increasing the area of the pin holes, but this can lead to cracking. Moreover, this drying process also results in considerable shrinkage of the wet gel.
In variations of this ambient pressure drying technique, colloidal silica particles have been added to the sol to increase the average pore size and to increase the strength of the solid matrix. Although this technique is generally effective, the presence of colloidal silica particles sacrifices the gel's otherwise inherent homogeneity, and thus restricts the range of compositions that can be utilized. In addition, devitrification spots can be created if mixing of the colloidal silica particles is imperfect.
Alternatively, drying control additives, such as dimethyl formamide, can be added to the sol, to enlarge the pores and to control the drying rate. These additives are then removed during the drying step. Although this alternative technique is generally effective in eliminating cracking, the resulting monoliths can sometimes have a large number of bubbles.
Another approach for eliminating cracking of the glass or ceramic gel during the drying step has been to hydrothermally age the gel while it is still wet. This increases the average pore size in the gel, and correspondingly decreases the capillary stresses encountered during drying. Although this technique is generally effective, the aging step increases the time and the equipment costs for drying gels.
Yet another approach for eliminating cracking of the gel during the final drying step is to dry the gel at an elevated temperature and pressure below the solvent's critical temperature and pressure. This subcritical drying process is carried out in a specially configured, sealed pressure chamber. The chamber is controllably heated, to evaporate the solvent and thereby cause the pressure within the chamber to rise until it eventually stabilizes at a substantially constant value. The value of this final pressure is determined according to the total amount of solvent, including both free solvent and solvent in the pores of the wet gel, present in the chamber before the chamber is sealed and heated. The chamber is sized so that it can accommodate all of this solvent in its gaseous form without reaching the solvent's critical pressure. This drying process is described in greater detail in U.S. Pat. No. 5,473,826, to Kirkbir et al. Although this subcritical drying process is effective in reliably and inexpensively drying wet gel monoliths, the limitation on the total amount of initial liquid solvent relative to the size of the drying chamber is considered to unduly limit the sizes of the gels that can be dried.
It should, therefore, be appreciated that there is a need for an improved drying process and apparatus such that the drying process can be carried out below the critical temperature and pressure of the drying solvent and that yields crack-free, porous glass and ceramic monolithic bodies with negligible shrinkage of the gel in even larger sizes than was previously attainable. The present invention fulfills these needs.